TGB Dual is an open source (GPLv2) GB/GBC emulator with game link cable support.

This is a port of it to libretro.  To emulate two units side by side, use
the core options in your frontend.  In older frontends, you'll need to use
your frontend's "Sufami Turbo" interface, placing the ROMs in "slots" A and B.
The Sufami Turbo BIOS itself can be any file; it is ignored by the emulator.
Use the Player 1 and Player 2 controllers to control each unit.

In addition, some of the Japanese comments in the source code have been run
through Google Translate to English, and CP932 and EUC-JP encoded files have
been converted to UTF-8.

Original sources from
http://gigo.retrogames.com/download.html#tgb-dual
http://shinh.skr.jp/tgbdualsdl/